(a) Technical Field
The present invention relates to a method for controlling battery State of Charge (SOC) in a hybrid electric vehicle. More particularly, the present invention relates to a method for controlling battery state of charge in a hybrid electric vehicle, which enables the efficient use of energy, the maximization of energy recovery by a motor, and the improvement of fuel efficiency and operability without the improvement of capacity and performance of electrical equipment (drive motor, Hybrid Starter and Generator (HSG), etc.) or a main battery in a hybrid electric vehicle.
(b) Background Art
Generally, internal combustion engine vehicles that use fossil fuels such as gasoline and diesel have a variety of limitations such as environmental contamination caused by the engine exhaust, global warming caused by carbon dioxide, and respiratory illnesses caused by creation of ozone. Accordingly, vehicles driven by electric power i.e., eco-friendly vehicles such as electric vehicles (EVs) driven by a motor and hybrid electric vehicles (HEVs) driven by an engine and a motor are being developed. In particular, electric vehicles and hybrid electric vehicles are equipped with a motor used as a driving source for vehicle running, an inverter and a motor controller (e.g., Motor Control Unit (MCU)), and a battery (usually referred to as ‘main battery’ or ‘high-voltage battery’) configured to supply a motor with driving power.
Additionally, electric vehicles and hybrid electric vehicles are equipped with a battery controller (e.g., Battery Management System (BMS)) configured to collect battery information. The battery controller is configured to collect battery information regarding the voltage, current, temperature, State of Charge (SOC) (%) of a battery, and is directly involved in charge and discharge control of a battery using the battery information or provides the battery information for other controllers within or extraneous a vehicle to allow the other controllers to use the battery information for the purpose of vehicle control or battery charge/discharge control.
Further, hybrid electric vehicles are equipped with an engine together with a motor (hereinafter, referred to as a ‘drive motor’) as driving sources, and a Hybrid Starter and Generator (HSG) power-transmittably connected to the engine to start the engine or generate electricity using power delivered from the engine. A main battery (e.g., high-voltage battery) used a power source of the drive motor is chargeably/dischargeably connected to the drive motor and the HSG via an inverter. The inverter is configured to convert a direct current of the battery into a three-phase alternating current (AC) for the driving of the drive motor, and apply the three-phase alternating current to the drive motor (e.g., battery discharge).
Such hybrid vehicles either drive in an electric vehicle (EV) mode which is a pure electric vehicle mode using the driving power of the drive motor, or in hybrid electric vehicle (HEV) mode which uses both driving powers of the engine and the drive motor. In addition, the regenerative mode that recovers braking or the inertial energy through the power generation of the motor during the braking of a vehicle or the coasting of a vehicle by inertia and charges power into the battery is performed. The HSG also charges the battery by operating as a generator by its own power of the engine or operating as a generator by the power delivered through the engine under the energy regenerative condition.
Meanwhile, in typical eco-friendly vehicles, the charge and discharge of a battery is adjusted based on the available output of the main battery and the required output necessary for current driving regardless of information of the vehicle speed and the road slope on the driving path. In particular, when a vehicle enters a uphill road or a low-speed section, or when a vehicle enters a downtown or a congested section and when the battery state of charge (hereinafter, referred to as ‘SOC’) (%) is low, an available motor torque is limited upon EV driving reduction and acceleration/deceleration, generating an increase of a transient control section and thus reducing the energy efficiency and operability (e.g., charge amount at idle and inefficiency operating point increases).
Further, when a vehicle enters a downhill road or a middle/high-speed section or when a vehicle enters a highway or an expressway, when the battery SOC (%) is high, an available batter space for charge is deficient upon regenerative braking and coast regeneration, making it difficult to recover extra regenerative energy and thus wasting the regenerative energy. Particularly, when there is frequent regenerative braking for maintaining a vehicle at a constant speed, or when a vehicle travels on a long-distance deceleration or downhill road such as the outlet of an expressway, regenerative energy may not be recovered and may be wasted.
In the related art, since the slope of a driving road is predicted from a signal of an accelerator (e.g., acceleration pedal) position sensor (APS), an output, a vehicle speed, or a signal of a G sensor (e.g., acceleration sensor) to determine a SOC control strategy or since the SOC control strategy is determined based on an APS signal, a brake pedal position sensor (BPS), or vehicle speed, an increase of a transient control section may occur and thus reductions of fuel efficiency and operability may be incurred due to misdetermination or determination delay according to variations (e.g., change of road load, change of acceleration or deceleration situation) of vehicle driving conditions during control section determination.
The above information disclosed in this section is merely for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.